1. Field
The present disclosure relates to a light-emitting device package, and more particularly, to the features of a sub-assembly of the light-emitting device package and/or of the package preventing encapsulant delamination.
2. Description of Related Technology
Although a person skilled in the art will appreciate that the concepts disclosed in this application are applicable to packages for semiconductor-based light-emitting devices, examples of which include, but are not limited to a light-emitting diode (LED) and a laser diode (LD), the state of related technology is explained using an LED as a typical example of a light-emitting device without any loss of generality, merely to avoid undue repetitiveness of the disclosure.
LEDs have been used for many years in various light emitting applications. Due to LEDs' advantages, i.e., light-weight, low energy consumption, good electrical power to light conversion efficiency, and the like, an increased interest has been recently focused on use of light-emitting diodes for high light intensity application, e.g., replacement of conventional, i.e., incandescent and fluorescent, light sources. To increase intensity of the light emitted by the light-emitting device if a design goal so requires, often more than one light-emitting die is arranged in a package. For the purposes of this disclosure a die has its common meaning of a light-emitting semiconductor chip comprising a p-n junction. Similarly, a package is a collection of components comprising a light-emitting device including but not being limited to: a substrate, a die or dice, encapsulant, bonding material(s), light collecting means, and the like. A person skilled in the art will appreciate that some of the components are optional.
FIG. 1 depicts a conceptual cross section of an exemplary light-emitting device 100 in accordance with known concepts. A substantially flat substrate 102 in addition to being a mechanical support is often used as a means for heat dissipation from the light-emitting device. When used in the latter function the substrate 102 is made from a material with high thermal conductivity. Such material may comprise metals, e.g., Al, Cu, Si-based materials, ceramics such as AlN, or any other material whose thermal conductivity is appropriate for the light-emitting device in question. A person skilled in the art will appreciate that material appropriate for a light-emitting device with power dissipation of, e.g., 35 milliwatts (mW) is different than material appropriate for a light-emitting device with power dissipation of, e.g., 350 mW. Flatness is understood to describe irregularities whose spacing is greater than the roughness sampling length. A material is considered to be substantially flat if the irregularities in flatness would not cause light to be reflected by such irregularities.
The source of light is at least one die 114, disposed on an upper face 104 of the substrate 102. Although four dice 114 are depicted in FIG. 1, a person skilled in the art will appreciate that such is for an illustration of the concept because the number of dice is a design decision, and one die may be sufficient should it satisfy design goals.
To improve light extraction from the light-emitting device 100, several measures are taken. First, surfaces that are transparent to photons emitted at a particular wavelength or that have poor reflectivity of such photons in an undesirable direction of emission may be treated, e.g., by polishing, buffing, or any other process, to acquire a specific reflectivity. Reflectivity is characterized by a ratio of reflected to incident light. Such surfaces are an upper face 104 of the substrate 102 and inner wall 106 of a support member 108. The support member 108 provides boundary for an encapsulant 110 and reflects light emitted by the die or dice 114 into desirable direction. Alternatively, the desired reflectivity is achieved by applying a layer of a material with high reflectivity, such as Ag, Pt, and any like materials known to a person skilled in the art, (not shown in FIG. 1) onto such surfaces.
Furthermore, to prevent reflection of the emitted photons from boundaries between materials characterized by different refraction indexes, and, consequently, loss of light intensity, an encapsulant 110 is applied into a cavity 112 surrounding the light-emitting region, i.e., the cavity created by the substrate 102, the support member 108, and the die or dice 114. The material for the encapsulant 110 is selected to moderate the differences between the refraction indexes of the materials from which components creating the reflective boundaries are made. In one aspect of the disclosure the encapsulant 110 is clear, i.e., comprising no fillers. However, the disclosed concepts apply equally to encapsulant 110 comprising fillers, e.g., phosphors.
Finally, certain light-emitting device packages further comprise a light-transmissive cover 116 disposed above the die or dice 114. Such a light-transmissive cover comprises e.g., a window for a protection of the inside of the cavity against environmental elements, or a lens for further focusing the emitted light. In order to prevent delamination of the encapsulant 110 from the surface of the light-transmissive cover 116 and/or the inner wall of the support member 108 and/or the die or dice 114 and/or the substrate 102, the light-transmissive cover 116 is allowed to float freely on the encapsulant 110, without being rigidly attached to the support member 108 with an adhesive or another fastening means. Such a configuration prevents significant residual stress, caused by temperature variation as the light-emitting device 100 heats and cools during the device's lifetime, to develop within the encapsulant 110. Because any delamination would introduce voids in the encapsulant, the resulting internal reflection optical losses caused by the above-described difference between materials characterized by different refraction indexes would cause loss of light intensity.
Although the configuration depicted in FIG. 1 may be suitable for LED packages comprising a clear light-transmissive cover, it is not particularly suitable for LED package comprising a light-transmissive cover coated or filled with phosphors; such a light-transmissive cover being often used for light conversion. An advantage of such a configuration is that the window or lens coated or filled with phosphors can be matched appropriately with a LED die or dice of known wavelength to achieve a more precisely controlled color corrected temperature (CCT). Different windows or lenses may have different phosphor coatings or fillings, and these matched with LED die or dice of optimal wavelength to achieve target CCT as needed.
However, a problem with this configuration arises from the fact that the temperature of the phosphor coated or filled light-transmissive cover increases significantly during operation of the light-emitting device because the conversion inefficiency of the phosphors results in generating significant heat. The increase in the temperature in turn results in decreased efficiency of the light-emitting device due to the decrees in light-conversion efficiency of the phosphors and decrease of efficiency of the die.
The above-described problem may be solved by a configuration according to FIG. 2, which depicts a conceptual cross section of another exemplary light-emitting device 200 in accordance with known concepts. The description of like elements between FIG. 1 and FIG. 2 is not repeated, the like elements have reference numerals differing by 100, i.e., reference numeral 102 of FIG. 1 becomes reference numeral 202 in FIG. 2.
Referring to FIG. 2, the main conceptual difference from FIG. 1 is that a light-transmissive cover 216 coated or filled with phosphors is attached to the upper face 218 of the thermally conductive support member 208. The bottom face 220 of the support member 208 is attached to a thermally conductive substrate 202. Thus, in this aspect, the support member further serves as supporting means for the light-transmissive cover 216. The light-transmissive cover 216, the support member 208, and the substrate 202 should be attached to one another using any thermally conductive means (not shown in FIG. 2) to maximize heat transfer between these components. By the means of example, such a thermally conductive means may comprise any thermally conductive adhesive or solder material, such as metal filled epoxy, eutectic alloy solder, and any other thermally conductive means known to a person skilled in the art. Furthermore, it is desirable that the light-transmissive cover 216 is also made from a thermally conductive martial. Such a configuration allows heat to flow from the phosphors through the window or the lens 216 and then through the support member 208 to the substrate 202.
A person skilled in the art will appreciate that in an alternative configuration; the light-transmissive cover 216 and the support member 202 do not need to comprise two separate components, but may be designed as a single component.
Since the heat from the light-transmissive cover 216 coated or filled with phosphors is now transferred to the substrate 202, proper heat dissipation from the LED package 200 must be assured to prevent loss of efficiency due to increased temperature of the die or dice 214. Such heat dissipation may be achieved by proper design of the above-described components of the LED package 214. In addition, the LED package 200 may further be attached to a suitable heat sink (not shown).
In any of the above-described configurations, the LED package 200 can operate without the phosphors or the LED die or dice over-heating beyond temperature that would significantly decrease the efficiency and/or reliability of the LED die or dice and the phosphors. A person skilled in the art will appreciate that the term significant describes a decrease in efficiency that would cause the light-emitting device performance fail to meet typical or minimum specification over the product life of the light-emitting device.
Although the configuration depicted in FIG. 2 solves the overheating problem, the above-alluded to problem of residual stress in the clear encapsulant 210 is re-introduced. The material commonly used for the clear encapsulant 210 is characterized by a relatively high coefficient of thermal expansion (CTE); consequently, the clear encapsulant 210 tends to undergo relatively high volumetric changes during the heating and cooling of the light-emitting device 200. In contrast, the cavity 212 undergoes only relatively small volumetric changes because the materials commonly used for components creating the cavity 212, are characterized by a relatively low CTE. A person skilled in the art will appreciate that the term “relatively” is used herein as disclosed infra. Examples of such commonly used materials are: silicone rubber, silicone gel (the clear encapsulant 210), Al, Cu, AlN (the substrate 202), Al, Ag plated Cu (the support member 208), sapphire, glass (the light-transmissive cover 216), light emitting semiconductor material (the LED die or dice 214). The disparity in the volumetric changes result in a significant residual stress within the silicone encapsulant during heating or cooling; thus potentially resulting in delamination of the encapsulant 210 from the surface of the light-transmissive cover 216 and/or the inner wall 206 of the support member 208 and/or the die or dice 214 and/or the substrate 202. As already mentioned above, such delamination can reduce the optical efficiency of the LED package, reducing the light intensity.
Accordingly, there is a need in the art for improvements in light-emitting device packages by providing means preventing the encapsulant delamination, increasing reliability and light extraction efficiency, and additional advantages evident to a person skilled in the art.